


On Strangest Tides: A One Piece Mermaid AU

by yukipri



Series: On Strangest Tides: A One Piece Mermaid AU [1]
Category: One Piece
Genre: Additional ship tags to be added, Art, Comic, Digital Art, Endgame Polyamory, F/F, F/M, Female Monkey D. Luffy, Gen, Genderbending, Headcanon, However this is generally an Everyone x Luffy AU you are warned, Mermaid Luffy, Multi, Polyamory, Roronoa Zoro/Monkey D. Luffy - Freeform, Thatch/Monkey D. Luffy, Vinsmoke Sanji/Monkey D. Luffy, Yami Yami Thatch
Language: English
Status: In-Progress
Published: 2020-11-22
Updated: 2021-02-05
Packaged: 2021-03-10 06:41:07
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 13
Words: 2,145
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27649021
Author URL: https://archiveofourown.org/users/yukipri/pseuds/yukipri
Summary: In which Luffy is a rubber mermaid who can't swim, she and Ace are co-captains on the same crew, and everything else ripples outwards from there.Posted as the "One Piece Mermaid AU" on Tumblr.＊This work contains the AU overview, illustrations, and comics. For fics in this AU, please check out the other works in this series!
Relationships: Monkey D. Luffy & Portgas D. Ace, Monkey D. Luffy & Portgas D. Ace & Sabo, Monkey D. Luffy & Sabo, Monkey D. Luffy/Everyone, Monkey D. Luffy/Portgas D. Ace, Monkey D. Luffy/Portgas D. Ace/Sabo, Monkey D. Luffy/Sabo
Series: On Strangest Tides: A One Piece Mermaid AU [1]
Series URL: https://archiveofourown.org/series/2021656
Comments: 90
Kudos: 279





	1. AU Overview

**Author's Note:**

> Hey there! If you're new to this AU, thanks so much for giving it a shot! If you've already been following this AU on my socials, thanks so much for coming to give it some love here <3
> 
> This AU has a slightly unconventional format, so lemme quickly explain for new folks. This AU is comprised of visual art, comics, text headcanons, and fics. All of these parts together build the story as a whole. It is also non-chronological, meaning that there is no single "main storyline," and it instead jumps around. 
> 
> To post on AO3, I've broken this AU into several parts based on media type. They will all be in this "Series," but will update simultaneously because they're not chronological continuations of each other. The parts are as follows:
> 
>  **1) On Strangest Tides: A One Piece Mermaid AU** \- This work! Contains the AU Overview (initial headcanons), art, comics, and other general info.  
>  **2) On the Ship's Log: A Collection of One Piece Mermaid AU Stories** \- A compilation of non-chronological shorter fics in this AU.  
>  **3) On the Courtship of Monkey D. Luffy: A One Piece Mermaid AU Story** \- A single longer fic containing some of the AU's shippier elements.  
>  **4) On Side Notes: A Collection of Ask Responses to the One Piece Mermaid AU** \- Ask responses from Tumblr that aren't fics but contain information relevant to the AU.

**Notes for the Chapter:**

> This was the headcanons post that started it all, and you can think of it as a general overview of the premise for this AU.

Luffy’s a mermaid who was raised in Fuusha village. Despite being a mermaid, she _sucks_ at swimming, a fact that Shanks finds absolutely hilarious. She still wants to become a pirate, she still eats her devil fruit and becomes a rubber mer(?), she still gets kidnapped by Higuma (who sees her as an excellent way to rake in cash, given how valuable mermaids are on the slave market), and Shanks still gives her his hat when she announces her intentions to become Pirate King.

As a devil fruit user, Luffy’s already atrocious swimming skills are now at zero. She can still breathe under water, but will sink like a stone, her strength sapped by the sea, and if she loses consciousness she’ll stop breathing too. Needless to say, she stays out of the water, and the villagers come to accept that sometimes, mermaids just live on land.

Garp comes back, and is still furious at her for wanting to become a pirate (and inwardly, extremely concerned that a bandit tried to catch and sell her), and still takes her to live at Dadan’s where she meets Ace.

Ace has no idea what to think of the weird fish-brat who he’s now told is gonna be his sibling, and resolutely ignores Luffy. He’s inwardly slightly impressed as Luffy continues to follow him through the mountain every day, not even particularly hindered by lack of legs and using a mixture of crawling, squirming, hand-walking, and ricocheting forward with rubber arms to move.

Luffy meets Sabo, and is still caught by Porchemy, who at first is thrilled because look, a mer brat! _That’ll bring in WAY more money than the spare change Ace stole!_ But in the end, he loses his temper at Luffy’s stubbornness and still beats her to a pulp.

Ace and Sabo still rescue Luffy, and they still exchange sake cups and become brothers.

Yes, _brothers_.

Because at this point, neither Ace nor Sabo nor any of the bandits, nor anyone really on Dawn island save Makino and Garp actually know that Luffy’s a girl. Not even Luffy herself.

The three brothers promise to each other that they’ll all leave the island when they turn seventeen, setting out to sea to become pirates. Ace and Sabo are honestly a little concerned, given how often people try to catch and sell Luffy, and how despite aquatic appearances, Luffy’s vulnerable in water...but they decide to wait and see what happens. They have seven years together, Luffy an additional three until seventeen, and who knows how much stronger their baby brother will get in that time.

Sabo “dies.”

Ace and Luffy continue to train, and things don’t change until Ace is seventeen, shortly before he sets out to sea.

He’s let Luffy crawl into his blanket to sleep with him (partially cold-blooded, Lu gets cold easily when inactive), and Ace realizes that his lil brother’s chest feels a bit lumpy.

Concerned, he makes Luffy have it checked out by the bandits, who are shocked to find developing…breasts?

That can’t be right… _right?_

Dadan makes a call to Garp, who snorts and says of _course_ Luffy’s a girl, didn’t he say she's his granddaughter?

No, he had not, he had only ever mentioned “grand child.”

Everyone is shocked, but none more so than Ace, who really doesn’t know what to do with this new information.

(Luffy’s not quite sure what a “girl” is, and when she asks Ace, Ace honestly doesn’t know how to define “girl” either, so she doesn’t get what the big deal is.)

Ace realizes he’s troubled because he can’t remember the number of times he’s rescued Luffy from potential traffickers, and the number of times he’s heard them lament, “Pity it’s not female, it’d be worth quadruple," before Ace beats the shit out of them.

He already had doubts about letting Luffy go out to sea alone, three years after he’s left. But now that he knows that Luffy’s a girl, and one of the most sought-after species that human traffickers target…he knows the world out there is much bigger than a teeny peaceful East Blue island, and Luffy may not get lucky every time, and may not be strong enough in just three years, or possibly ever, to ever be truly safe.

But at the same time, it’s not like he can order her to stay here. There’s no way Luffy’d listen; the sea calls to her, freedom calls to her, and Ace understands that more than anyone.

_Sabo_ , Ace thinks, _what would you do?_

The night before he leaves, Ace tells Luffy that there’s been a change of plans.

He’s still leaving for sea. He’s going to get stronger, and work his way towards becoming Pirate King.

BUT, three years later, Luffy will _not_ leave Dawn Island alone.

Ace promises that he’ll come back, a stronger pirate capable of looking out for his baby brother (because girl or not, they exchanged vows of brotherhood, and that’s something that can’t be changed, with Sabo gone).

He refuses to let Luffy be his captain, he’s still got his pride, but maybe, _maybe_ if Luffy becomes strong enough, he’ll let her be Co-Captain.

Luffy is thrilled, because while she was prepared for them to go their separate ways, pirating with Ace would be the best! She grudgingly accepts the compromise, and promises to train and wait for Ace’s return.

Ace leaves Dawn island, and makes a name for himself on the Grand Line (and he’s PISSED when he finds out he’s eaten a devil fruit, because now how’s he supposed to rescue Lu from drowning when he can’t swim himself???).

He eventually makes it to Whitebeard, and eventually comes to admire the man. Whitebeard invites him to his crew, and Ace honestly replies that a large part of him wants to accept…but he can’t. He promised his baby brother that they’ll be pirates and co-captains together, and he can’t join another pirate crew without Luffy agreeing too.

Even so, and he knows it’s selfish of him, he still wants to call Whitebeard his father.

Whitebeard tells him that Ace is already his son, regardless of where his allegiances lie, and gives him his blessing to return to East Blue to fetch Luffy. Whitebeard laughs that he can’t wait to meet the lil brat that Ace speaks so highly of, and for Ace to hurry up and come back to the New World so they can meet.

Ace plans on traveling light, his former crew all choosing to join the Whitebeard pirates except for his first mate, Deuce. Ace loves his crew, but they’re also _his_ crew, and he knows that he wants to build _their_ crew with Luffy. So he thinks he and Deuce will be a good starting point ( _You’re just bringing me along because you want someone who can swim on the crew_ , Deuce accuses).

As they begin preparing to leave, Marco lets slip to fellow commander Thatch that he heard that Ace’s “little brother” is actually a super cute mermaid girl.

The next morning, Thatch shows up too, insisting that he come along, because hey! He’s been with the Whitebeards for ages and hasn’t been to Paradise in a while, he wants some change! And won’t it be nice to have someone as reliable as him along, just until they get back to the New World and reunite with the Whitebeards? Really he has no ulterior motives like wanting to check out Ace’s hyper hot baby brother!

Ace is suspicious, but Thatch is already on board and the other Whitebeards are already waving, so he lets it go.

(Shortly after they leave, Thatch discovers a devil fruit. Deuce tells him to sell the damn thing, it’ll taste like shit, but Thatch thinks, _boy wouldn’t it be great to be able to woo Ace’s hot brother with a really cool devil fruit power?_ So he eats it, and yeah it tastes like shit, but now he can control Darkness which is sorta badass?)

(Somewhere on the Whitebeard ships, Blackbeard is still waiting for someone to find the darkness devil fruit, and well, it doesn’t happen.)

Before Ace returns to Dawn Island, part of him still hopes that Luffy looks passably like a guy. Having been to Fishman island, having befriended Jinbe, and having personally seen the dangers mermaids face...He knows that mermen can still be targeted, but mermaids are still exponentially more vulnerable, and it's a risk he hopes to avoid exposing Luffy to. He’s learned the kinds of clothing that can help conceal tails, and hopes that there’s still a slim chance that they might be able to be pirates without the world knowing that Lu’s a mermaid.

His hopes are shot when Luffy rockets into his arms at the dock, and his face is immediately buried in enormous tits that definitely weren’t there three years ago.

Despite Ace’s growing concerns (and red face; _why does he feel so hot? It’s not his devil fruit…_ ), Ace lets Luffy give Deuce a beating ( _why me?!_ ) to claim her position as Ace’s Co-Captain of the newly established ASL pirates ( _Because if we’re gonna be pirates together, Sabo has to be with us too!_ ).

Luffy’s gotten a lot stronger, but is still utterly ignorant of the outside world and the dangers it holds (such as the drooling Thatch who immediately wins Luffy over with his cooking despite Ace’s burning glares). She’s reckless and falls into the ocean every damn day, and while Deuce and their other new crew members dive in after her and they’ve discovered her useful ability to talk to fish to ask for help, it still gives Ace heart attacks.

They pick up more crew members while in East Blue, including Roronoa Zoro, Usopp, and Sanji, a pervy cook who competes daily with Thatch for Luffy’s affections through food (Luffy appreciates the food).

At Arlong Park, Luffy encounters fishmen for the first time in her life. Arlong mocks Luffy’s choice of friends and family, and invites her to join his crew made of her own kind. Fishmen and mer are the superior race, and Luffy will make a fine wife, Arlong says.

Luffy breaks his nose off, and Nami joins their crew.

Luffy’s chosen mode of transportation is on the back of one of her sturdier crew mates, usually Ace, but often Zoro too. She unfortunately hates all the long skirts and robes Ace suggested for her to hide her tail, and so Ace has decided that being as intimidating as possible while carrying his brother glued to his back is the only way to go.

Needless to say, news of the former Spade Pirates Captain and Whitebeard Commander Thatch ( _are they an extension of the Whitebeards??_ ) establishing the new ASL pirates spreads fast. That, and rumors that there’s a beautiful young mermaid on board who can’t swim. Despite the strength of the crew and their attempts at vigilance, Luffy’s still dumb and gullible, and is kidnapped (and rescued) a dozen times before they’re even on the Grand Line.

(After a few times, Luffy’s uncharacteristically quiet, and privately asks Ace if they can break up the crew. She’s holding Ace back; she not only made him come back to her, but half their adventures just seem to be her crew rescuing her. She’s strong enough to defeat enemies, but not save herself. Ace tells her to shut up, and never bring this up again; they’re _brothers_ , and he’s not going to leave her behind or make her abandon her dreams when she can’t accomplish them alone, and that’s his choice. Luffy doesn’t bring it up again, but works harder than ever to become stronger and earn recognition as Ace's Co-Captain when the world seems intent on thinking of her as his pet.)

During one of these kidnappings, Luffy’s successfully brought all the way to an auction house before her crew can rescue her. An auction house that unbeknownst to them, was under investigation by the Revolutionary Army.

Liberating the enslaved is an unfortunately common mission for them, and Sabo’s doing final sweeps as Koala frees the last of them when he notices a tank stowed away in a corner. He’s familiar with these tanks, used to showcase mermaids on the rare occasions one can be caught, and it looks empty but he’s still drawn to it for some reason.

It’s only when he gets closer that he notices a mermaid crumpled at the bottom of the enclosure. She’s completely slumped over, and he’s afraid she’s already gone, even as something about her screams with familiarity.

He’s cracked the tank and has reached in to pull her out, when the wall behind him explodes in flames, revealing a furious Ace who snarls at the strange man with his hands on his baby brother…

…and then Sabo’s memories come back.

**Notes for the Chapter:**

> Thanks so much for reading and for giving this a shot! Any feedback, comments, kudos, encouragement, anything at all is greatly appreciated!!!
> 
> As mentioned at the beginning notes, this AU has been posted in advance on my social medias, and I'm just slowly compiling it here. If you would like to see the all currently posted works, please check out my [**TUMBLR**](https://yukipri.tumblr.com/) or [**TWITTER**](https://twitter.com/YukiPri_Art) (twitter contains just art)!
> 
> And while I adore any comments anywhere, if you would like to send me ask prompts for this AU, I'm still accepting them on Tumblr, but I'd really appreciate if you could first check out my already posted asks and guidelines, which you can view [**HERE**](https://yukipri.tumblr.com/post/625291826616827904/one-piece-mermaid-au-text-headcanons-compilation)!
> 
> Thanks so much!  
> Love,  
> ~YukiPri (Kazu)


	2. [COMIC] The Merboy(?) who can't swim

**Summary for the Chapter:**

> In which Luffy is a Rubber Mermaid who can’t swim

**Notes for the Chapter:**

> ＊ Read comics Japanese direction, Right to Left
> 
> ＊Difference in pronouns between the 2 comics is intentional. Luffy isn’t being misgendered; no one, including Luffy herself, knew she was a girl until she was 14, and she’s cool with (or rather doesn’t care either way) the label rn.

**Notes for the Chapter:**

> Double chapters to start off! Any feedback, comments, kudos, encouragement, anything at all is greatly appreciated!!!
> 
> As mentioned at the beginning notes, this AU has been posted in advance on my social medias, and I'm just slowly compiling it here. If you would like to see the all currently posted works, please check out my [**TUMBLR**](https://yukipri.tumblr.com/) or [**TWITTER**](https://twitter.com/YukiPri_Art) (twitter contains just art)!
> 
> And while I adore any comments anywhere, if you would like to send me ask prompts for this AU, I'm still accepting them on Tumblr, but I'd really appreciate if you could first check out my already posted asks and guidelines, which you can view [**HERE**](https://yukipri.tumblr.com/post/625291826616827904/one-piece-mermaid-au-text-headcanons-compilation)!
> 
> Thanks so much!  
> Love,  
> ~YukiPri (Kazu)


	3. [ILLUST] Luffy on Ace's back

**Summary for the Chapter:**

> Luffy's main mode of transportation is hanging off of Ace's back. That this also allows Ace to intimidate people who look too long at his baby brother is a convenient side effect.


	4. [ILLUST] Ace has a new hat

**Summary for the Chapter:**

> Ace has a new hat, 'nuff said.

**Notes for the Chapter:**

> Does this work require a "big boobs" warning, squints....


	5. [COMIC] My Little Brother is a Mermaid (♀) Part 1

**Summary for the Chapter:**

> Shortly before Ace is due to set sail from Dawn Island, some discoveries are made.

**Notes for the Chapter:**

> Read Right to Left, Japanese direction

**Notes for the Chapter:**

> As always, thank you so much for reading! And comments at all are immensely appreciated!!! And also, please check out the other works in this series if you're interested in fics in this AU, all 3 other parts are updating simultaneously!


	6. [COMIC] My Little Brother is a Mermaid (♀) Part 2

**Summary for the Chapter:**

> Continuation of the previous chapter!

**Notes for the Chapter:**

> Read Japanese word order, Right to Left!

**Notes for the Chapter:**

> As always, thanks so much for reading! This part was admittedly meant to be more informative than entertaining, and hopefully answers any questions regarding Lu's gender and her choice to remain "Little Brother."
> 
> If you enjoyed, please do consider leaving a comment or kudos! Thank you!!


	7. [ILLUST] Luffy & Sabo

**Notes for the Chapter:**

> Happy holidays, to those who celebrate!


	8. [ILLUST] Luffy gets a new outfit

**Summary for the Chapter:**

> 'Nuff said, Lu gets a new outfit.

**Notes for the Chapter:**

> Digest notes:
> 
> Nami chose Lu’s clothes, and not just bc she genuinely likes dressing up her captain in cute stuff, but also bc Ace pays for the clothes, and the more gobsmacked he is by how cute his lil bro is, the less likely he is to notice her making him pay for Nami’s clothes too…^ ^;
> 
> The key items Nami chose this time are the lil red beaded accessories that match Ace’s necklace/hat beads, appealing to Ace’s subconscious possessive side.
> 
> Lu herself doesn’t quite get why her dressing up excites everyone so much, and she’s sorta annoyed about the fabric and jingly stuff getting in the way of her tail…BUT it makes her crew happy, which makes her happy, so she’s ok with it!

**Notes for the Chapter:**

> Happy New Year! May 2021 be kind to us all....Thanks so much for stopping by this work, and and thank you again to everyone who drops me a kudos/comment <3


	9. [ILLUST] 3 bros childhood

**Summary for the Chapter:**

> When they were young...

**Notes for the Chapter:**

> Ngl, between major art burnout and work and whatever the heck's going on in the US this week, i just completely forgot to update anything ^ ^;;; Here's to my attempts to get back to posting schedule starting today.
> 
> As always, I'm super grateful for any comments you can spare!


	10. [ILLUST] Ace & Zoro rescue Luffy

**Summary for the Chapter:**

> In which Ace and the crew rescue Luffy (again) from human traffickers. Ace may be smiling, but he’s not smiling.

**Notes for the Chapter:**

> Aaand we're slowly catching up to the illustrations I have done for this AU...
> 
> As always, thank you so, so much to those of you who can spare a kudos or comment! m(_ _)m


	11. [COMIC] Big Brother's Back

**Summary for the Chapter:**

> The origin of how Luffy came to ride on her brother’s (brothers’) back.

**Notes for the Chapter:**

> Read Japanese direction, right to left!

**Notes for the Chapter:**

> The last of the comics that I currently have finished, and this was posted...August 2020. *SWEATS* Here's to hoping I actually finish any more...
> 
> As always thank you so much for viewing, and if you enjoyed this, any comments are immensely appreciated!!! m(_ _)m


	12. [ILLUST] Deuce Sketches

**Summary for the Chapter:**

> Some sketches of Deuce, the first mate of the ASL Pirates! Because there aren't enough official images of him!

**Notes for the Chapter:**

> Note: Deuce, aka Masked Deuce, is not an OC. He's a character in the two Ace novels, Novel A, and a former member of the Space Pirates, and was Ace's first mate when he was captain.
> 
> Also, while this is pretty gen art and doesn't techincally _need_ to be specific to this AU, I did specifically draw it with the intention of it being a reference for this AU...so into this AU's art it goes LOL!

**Notes for the Chapter:**

> Thanks so much for viewing!!! <3 Comments make me feel warm and fuzzy inside~


	13. [ILLUST] Thatch sketches

**Summary for the Chapter:**

> Some Thatch sketches! Because he spends a lot of time with his hair down in this AU, which doesn't happen in canon ^ ^;
> 
> Also, a sketch of Yami Yami!Thatch (which is best Thatch in my opinion ;D)

**Notes for the Chapter:**

> Thanks for viewing! Comments appreciated <3

**Author's Note:**

> Thanks so much for reading and for giving this a shot! Any feedback, comments, kudos, encouragement, anything at all is greatly appreciated!!! (I have a lot of backlog for this, and posting schedule is determined by feedback ;D)
> 
> This AU has been posted in advance on my social medias, and I'm just slowly compiling it here. If you would like to see the all currently posted works, please check out my [**TUMBLR**](https://yukipri.tumblr.com/) or [**TWITTER**](https://twitter.com/YukiPri_Art) (twitter contains just art)!
> 
> Also, while I adore and am incredibly grateful for any comments anywhere, if you're interested in asking me longer questions regarding this AU or prompts for Ship's Log/Side Notes, I'm still accepting them on Tumblr! But, I'd really appreciate if you could first check out my already posted asks and guidelines, which you can view [**HERE**](https://yukipri.tumblr.com/post/625291826616827904/one-piece-mermaid-au-text-headcanons-compilation)!
> 
> Thanks so much!  
> Love,  
> ~YukiPri (Kazu)


End file.
